The proposed research involves the development of cationic psi-cyclization reactions as a general methodology for synthesis of compounds containing spirocyclic ring junctions. Preliminary studies show that certain cyclizations of this type proceed with high stereo-selectivity. The initial studies will concentrate on the formation of a variety of spiro (4.5) decane and spiro (5.5) undecane systems from cyclohexenyl or cyclopentenyl cation type intermediates. The results of these studies will be applied to the development of synthetic approaches to several structurally and biologically interesting natural products including perhydrohistrionicotoxin, a neurotoxic alkaloid useful for study of neuromuscular phenomena, spiro (4.5) decane sesquiterpenoids, aphidicolin, a diterpenoid antibiotic, and gascardic acid, a novel sesterterpenoid.